The present invention relates to the technical field of composites (compounds), in particular composites having carbon fibres as preferably particulate additives.
In particular, the present invention relates to carbon fibre-containing particles, in particular in the form of pellets or granules, which are, in particular, suitable for producing carbon fibre-containing materials, in particular carbon fibre-containing plastics, in particular carbon fibre-containing composites, and a process for producing them and also their use.
Composites (also referred to synonymously as compounds or moulding compositions) are generally preferably homogeneous mixtures of, in particular, a single type of base materials, in particular plastics and plastic mixtures, to which at least one aggregate, e.g. fillers, reinforcing materials or additives, has been added in order to improve the properties of the base material, in particular in respect of improved performance, reduced costs, simplified further processing and more attractive appearance. Thus, for example, the UV stability and the hydrolysis resistance of plastics which are to be used for components in the exterior sector can be improved significantly by addition of stabilizers.
As fillers, use is made of, in particular, particulate materials such as talc, chalk, mica, barium sulphate, carbon black, ceramic powder, metal powder or the like, which are intended to increase the volume of the composites without changing the important properties. An increase in the volume is desirable with a view to, in particular, the storage and transport costs for the composites.
The addition of additives such as antioxidants, lubricants, antistatics, stabilizers, light stabilizers, parting agents, mould release agents, nucleating agents, UV absorbers, flame retardants, fluoropolymers (e.g. PTFE), pigments or the like allows the properties of the base material to be set in a manner targeted at the respective application. Additives enable, for example, the colour or the thermal and chemical stability of the composite to be adjusted. Furthermore, the addition of processing aids leads to improved processability of the composites in subsequent processes such as injection moulding or the like.
Changes in the mechanical properties of the base material, in particular impact modification of mechanical parameters such as tensile strength, elongation at break and impact toughness, can be achieved, in particular, by the addition of reinforcing materials. Reinforcing materials used are, in particular, glass fibres, preferably in the form of short and long glass fibres, carbon fibres or wollastonite.
Composites are produced, in particular, by compounding, where compounding corresponds to the treatment of the plastic and describes the upgrading process of plastics by mixing in aggregates (e.g. fillers, reinforcing materials, additives, etc.) to achieve targeted optimization of the properties of the base materials. In general, compounding encompasses various process stages or process operations such as transport, melting, dispersing, mixing, degassing and pressure buildup, and is carried out, in particular, in extruders, for example corotating twin-screw extruders, contrarotating twin-screw extruders, planetary-gear extruders, cokneaders, etc.
To achieve very homogeneous mixing of base materials with aggregates, the aggregates should be able to be incorporated very homogeneously into the base material during compounding. However, most of the abovementioned aggregates have, as a result of their poor wettability, often only unsatisfactory incorporability, so that the result is composites which are not fully homogeneous and, owing to the inhomogeneous distribution of the aggregates, do not have constant properties within the composite and thus only unsatisfactory quality.
Owing to the poor incorporability of the aggregates, demixing of base material and aggregates can also occur, so that production of the composite is not readily possible. This can, in particular, be the case when large amounts of various aggregates are used, in particular when the aggregates have significantly different densities from the base material, in particular the plastic.
Although the poor incorporability of the aggregates can be improved by the use of high shear forces, partial destruction, in particular comminution, of the aggregates can occur here. Particularly when using stiffening materials, comminution leads to significantly poorer mechanical properties of the composite.
Furthermore, the incorporation of aggregates in the compounding operation often leads to an adverse effect on the rheological properties, in particular the flow properties or flow rates. However, a deterioration in the flow properties, in particular in the case of large amounts of aggregates, is disadvantageous for further processing of the composites since high flow rates are required for the subsequent processing of the composites, e.g. in injection-moulding apparatuses or the like.
The aggregates also have poor handlability because of their often dust-like nature. Thus, special extraction and filter units have to be used in the case of dust-like aggregates in order to rule out a hazard to health during the production and use of the aggregates as a result of dust formation and to avoid the risk of dust explosions. Furthermore, owing to the dust-like nature, the meterability of the aggregates is also poorer since metering of an exact amount of aggregate or production of an exact mixture of aggregates is not ensured. However, exact meterability of the aggregates is of great importance for constant quality of the composites.
To achieve better incorporability, handlability and meterability of the aggregates, shaped bodies, in particular agglomerates, pellets, briquettes or the like, are sometimes also produced from the relevant aggregates in the prior art. However, the shaped bodies produced according to the prior art do not give satisfactory results since they often do not enable sufficiently homogeneous incorporation into the base materials, in particular plastics. The incorporation of the shaped bodies known from the prior art into base materials (e.g. plastics) also mostly leads, particularly in the compounding operation, to an adverse effect on the rheological properties, in particular the flow properties, in particular in the case of large amounts of aggregates, so that the result is composites which can be processed further only with difficulty because their low flow rate.
Furthermore, owing to the high proportion of the auxiliaries (e.g. sizes, etc.) used for producing the shaped bodies in the prior art, flaws can occur within the base material matrix, in particular polymer matrix, which can lead to defects within the composite; when using the shaped bodies containing the auxiliaries as per the prior art, incompatibilities with the base material used can also occur, so that the shaped bodies can be used only for very particular base materials and cannot be used universally for a large number of base materials.
The shaped bodies produced from aggregates in the prior art are consequently associated with many disadvantages in relation to their industrial use, so that there is an increased demand for shaped bodies composed of aggregates which have improved incorporability into a variety of base materials, in particular plastics, with the rheological properties of the resulting composite not being significantly impaired.